Your desire to lie around the house isn't because you're a slacker: it's in your genes. New research from scientists at the University of North Carolina shows that there's finally a good reason why some people would rather read comic books and play video games all day than, say, go run a marathon. According to kinesiologist Timothy Lightfoot and colleagues, there's a set of at least 23 genes that control the drive to be physically active in mice. Though he's yet to run the same genetic tests on humans, Lightfoot says he has reason to believe it will hold true for us, too.

According to a release on Lightfoot's work:

"Can you be born a couch potato? In exercise physiology, we didn't used to think so, but now I would say most definitely you can," said Lightfoot.

Of course, loungers don't get off the hook entirely — Lightfoot's study showed that only half the difference between highly active mice and lazier mice could be attributed to their genetics. So an animal's environment — in people's case, whether they live in a 4-story walk up, or dig ditches for a living — is going to have a big impact on how active they are. But Lightfoot says the evidence is piling up that there's more to being lazy than we thought.

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Subsequent studies have led the team to suspect that genetic differences are having a profound affect on mouse activity levels by causing significant differences in their brains.

"More and more what we are seeing is differences in brain chemistry. We are really convinced now that the difference is in the brain," Lightfoot said. "There is a drive to be more active."